Invariant.group and mustalias docs fixes

Summary:
Alias analysis would like to know that
invariant.group.barrier returns pointer that mustalias,
but this can't imply that we can replace one pointer with another

Reviewers: dberlin, sanjoy

Subscribers: llvm-commits, chandlerc, hfinkel, nlewycky, amharc

Differential Revision: https://reviews.llvm.org/D31758

llvm-svn: 300033